EKG (electrocardiogram) measuring apparatuses are primarily used for measuring and monitoring the action of a patient's heart. To this end typically the summation voltage of the electrical activity of the myocardial fibers is measured as an “EKG signal” via at least two electrodes. FIG. 1 illustrates by way of example an ideal curve of such an EKG signal as a voltage U over time. Characteristic curves of the EKG signal are characterized in accordance with Einthoven by the letters P, Q, R, S and T and usually reproduce the different phases of a heartbeat.
In addition to simply monitoring the action of a patient's heart there are other applications. For example, EKG signals are also used in medical imaging for generating trigger signals. The EKG signal is used during imaging to obtain information about the cardiac phase, in order thus to synchronize the imaging with the activity of the heart. In particular in imaging methods that require a longer recording time, high-quality cardiac recordings or even recordings of regions that are moved by the heartbeat can be created.
EKG measuring apparatuses are also used during an examination of a patient by means of a magnetic resonance device, for example for in-situ recording of EKG signals. In this case however the operation in the magnetic resonance device places special demands on the EKG measuring apparatus because of the strong gradient fields and high-frequency fields used there for imaging, in order to prevent mutual interference between magnetic resonance device and EKG measuring apparatus. EKG measuring apparatuses that are magnetic-resonance-compatible in the aforementioned sense are available on the market.
The determination of R waves in EKG signals is essential for reliable triggering. However, this determination is made more difficult e.g. by T wave elevations occurring in the magnetic field. A further major ongoing problem for reliable EKG signal measurement is temporally changing magnetic fields, as are used in the magnetic resonance device as magnetic gradient fields for spatial encoding. Such temporally changing magnetic fields generate interference voltages according to the law of induction, which are coupled in as interference in the EKG signal recorded by the EKG electrodes. Such magnetically generated interference signals overlap with the EKG signal generated by the heart and distort it.
These interferences are highly undesirable. To synchronize a recording of a magnetic resonance image with the heartbeat it is necessary to reliably identify the R wave of the EKG signal. The interference signals can, e.g. because of their often similar shape, be erroneously interpreted as an R wave and thus incorrectly trigger a recording of a magnetic resonance image. On the other hand it can happen that a “genuine” R wave is not identified as such because of the overlaid interference signals. This regularly leads to a significant worsening of the image quality.